I. Field of the Invention
The present invention relates generally to bandpass filters. In particular, the present invention is a planar bandpass filter.
II. Related Art
Filters are essential to the operation of most electronic circuits. Filters are implemented in electronic circuits to alter the amplitude and/or phase characteristics of a signal with respect to frequency. The design of bandpass filters for RF receivers/transceivers requires numerous tradeoffs with respect to circuit topology, bandwidth, clement realization, insertion loss, rejection level, etc. to arrive at a bandpass filter that satisfies all requirements. RF receivers/transceivers require bandpass filters that provide minimum insertion loss over the desired frequency band. In many cases, the design of conventional filters is driven by the range of capacitor values over the frequency band of interest.
Existing ceramic filters do not provide the rejection levels required by many RF applications without cascading the ceramic filters with other types of filters. Cascading filters results in increased insertion loss levels, and higher costs.
What is desired is a bandpass filter that does not require cascading ceramic filters to obtain the rejection level specifications needed. What is also needed is a bandpass filter that provides a repeatable design that does not require sampling and testing of multiple lumped element passive components such as capacitors or other filters, or tuning of the capacitors to achieve the desired performance levels of the filter. What is needed is a planar bandpass filter topology that provides a broadband response that eliminates reentrance frequencies at the desired rejection bands, while meeting the other requirements above.
One type of filter that satisfies some of these requirements is the combline filter. It can be realized in a planar form, is compact in size, and can be designed to provide a desired broad-band rejection characteristic.
Combline filters, however, have other problems associated with them. These problems arise when the production of mass quantities of filters is required, one of which is the use of lumped element capacitors in a conventional realization of the filter. Each filter usually includes a plurality of lumped element capacitors. Capacitor values vary over lot size. Variations in capacitor values can result in varied filter performance. To remedy the variation in capacitor values, a large quantity of capacitors could be tested to select capacitors with values that fall within a certain tolerance. Another alternative would be to build more filters than are needed, test each filter, and discard those filters that do not meet specifications. These remedies are very time consuming as well as expensive. Slight variations in the placement of the capacitors from filter to filter can also affect filter performance. Slight variations in filter topology from filter to filter, such as variations in tap points for tapped input and output resonators, can cause broadband response reentrance frequencies to occur at undesired harmonics of the passband.